The present invention relates to an absorbent article, and description will hereinafter be made as regards a sanitary napkin as a representative example of the absorbent article since the same idea can basically apply to other absorbent articles.
A sanitary napkin generally utilizes, as basic constituents, three components, namely a liquid-permeable surface sheet, an absorbent layer made of fluff pulp, absorbent tissues, a water-absorbent polymer or the like, and a liquid impermeable-leakproof sheet. Recently, a sanitary napkin comprising a rayon fiber assembly (hereinafter referred to briefly as "cushioning wool") or the like provided under the surface sheet (on the absorbent layer) has been generally used.
An object of the present invention is to greatly improve the defects of the absorbent layer and cushioning wool which have been generally used in a conventional sanitary napkin or the like as described hereinbefore.
First, fluff pulp is economical, flexible and highly hydrophilic. Thus, it is widely used as the absorbent layer.
Since, however, water absorbent by a laminate of fluff pulp is nearly completed based on a retention of liquid between fibers (capillary) and the laminate has no interfiber bonding strength, it is easily deformed and capillaries formed between the fibers are broken so that liquid retained is easily released when it undergoes a compressional force. The laminate shows a little recovery from compression when it is dry, but no recovery when it contains absorbent water. This causes slippage and wilting of the absorbent layer, and hence constitutes the main cause of the occurence of leakage. The following techniques for eliminating the above-mentioned defects have been disclosed.
(1) As shown in FIG. 4, a core material 4 having a shape retention property, such as a foam, is provided in the absorbent layer 3 or on the side of the non-effective surface to prevent the deformation of the absorbent article (see Japanese Utility Model Laid-Open No. 6,099/1981)
(2) As shown in FIG. 6, the absorbent layer itself is replaced with an elastic porous material 5 (e.g., a urethane skeleton foam, a polyester fiber assembly having fibers bonded with a binder, or the like) (see Japanese Patent Laid-Open No. 22,354/1986).
In the case of (1), however, although the shape retention property of the construction as a whole can be maintained, when the absorbent layer itself is deformed slippage and wilting are caused as shown in FIG. 5.
In the case of (2), although the recovery from deformation of the absorbent article is excellent, the hydrophilic nature of the elastic porous material is so extremely poor as compared with the fluff pulp that the material is not sufficient in liquid uptake capacity and liquid retention.
Since a cushioning wool 6 as shown in FIG. 7 is used mainly for the purpose of effectively absorbing a liquid excreted on the surface sheet 1 in the inside and causing the liquid to migrate into the absorbent layer 3, it is generally made of rayon fiber (occasionally pulp). When the amount of excretion is relatively small, it is very effective. In contrast, when the amount of excretion is large, the cushioning wool wilts due to its highly hydrophilic nature just as in the case of the above mentioned fluff pulp. As a result, a layer having a large amount of a liquid absorbed therein is formed between the absorbent layer 3 and the surface sheet 1, and the liquid readily flows back to the surface sheet, thus leading a to lowering in dryness and causing a side leak.
Various improvements of these defects have been proposed. For example, a method has been proposed to use a synthetic fiber instead of a rayon fiber, and another method use a fiber assembly (tow). Since the synthetic fiber and the fiber assembly both have a low hydrophilic nature, however, uptake of a liquid from a surface sheet is insufficient, and the liquid inevitably diffuses between the surface sheet and the cushioning wool, thereby causing poor dryness and a side leak.
Since no sufficient improvement in the resistance to compression is attained as well, no improvement is made as regards prevention of the ready back flow of a liquid under a pressure in the direction of compression due to wearing.